deps: update dependency protobuf to v3.20.3 (#120)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [protobuf](https://developers.google.com/protocol-buffers/) | `==3.19.5` -> `==3.20.3` | [![age](https://badges.renovateapi.com/packages/pypi/protobuf/3.20.3/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/protobuf/3.20.3/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/protobuf/3.20.3/compatibility-slim/3.19.5)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/protobuf/3.20.3/confidence-slim/3.19.5)](https://docs.renovatebot.com/merge-confidence/) |

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.kokoro/requirements.txt

@@ -162,9 +162,9 @@ google-cloud-core==2.3.1 \
     # via
     #   -r requirements.in
     #   google-cloud-storage
-google-cloud-storage==2.5.0 \
-    --hash=sha256:19a26c66c317ce542cea0830b7e787e8dac2588b6bfa4d3fd3b871ba16305ab0 \
-    --hash=sha256:382f34b91de2212e3c2e7b40ec079d27ee2e3dbbae99b75b1bcd8c63063ce235
+google-cloud-storage==2.0.0 \
+    --hash=sha256:a57a15aead0f9dfbd4381f1bfdbe8bf89818a4bd75bab846cafcefb2db846c47 \
+    --hash=sha256:ec4be60bb223a3a960f0d01697d849b86d91cad815a84915a32ed3635e93a5e7
     # via
     #   -r requirements.in
     #   gcp-docuploader